Increased difficulty and cost of obtaining and processing fossil fuels as well as concern over pollution caused by their use has increased interest in alternative sources of power, especially from so-called renewable resources such as hydroelectric power, wind-powered generators and solar power collection panels, generally referred to simply as solar panels. Of these possible power sources, solar panels involves the least expensive infrastructure and initial capital expense and are most easily scalable for use by private individuals and small communities. For example, while relatively large so-called solar farms may be installed to commercially produce relatively large amounts of power, many private residences have a roof portion that is properly oriented for solar power collection and of sufficient area to collect a significant fraction of the total power required for the residence. The cost of solar panels to cover such an area may be as small as several thousand dollars and such a cost can generally be recovered through reduction of commercially purchased power over a relatively small number of years.
However, such residential installations of solar panels are often subject to shading from surrounding trees, adjacent roof areas and other structures such as chimneys and the like. Since solar panels are generally comprised of a plurality of strings of series-connected photovoltaic (PV) cells, the shading of a very small number of individual PV cells can have a substantial effect on the total power that can be harvested from the panel. The voltage and current that are developed by a PV cell are a function of the light flux incident on the cell and the temperature of the cell. Therefore, when one or more PV cells in a series connected string are shaded or even a single PV cell partially shaded, both voltage and current of the entire string are reduced. Since available power is the product of the voltage and current produced, the reduction of power due to shading of even a single cell in a solar panel is significant.
In an effort to increase the amount of power that can be harvested, so-called maximum power point tracking (MPPT) control that involves use of a power converter to control the voltage input from the solar panel to the power converter. Since the load to which the power converter delivers power is effectively a power sink that consumes or stores all power delivered to it, the power converter can adjust the input voltage to itself from the PV cell or solar panel. At power levels below the maximum power available from a PV cell an increase in input voltage will cause an increase in current and power delivered. However, an increase in input voltage above the input voltage corresponding to maximum deliverable power can cause a reduction in current and power delivered. Therefore, MPPT control schemes cause a periodic perturbation in control of the input voltage to the power converter to determine if the input voltage is below or above that which will deliver maximum power and the power converter control can be adjusted such that the input voltage to the converter will seek, find and track the input voltage which will deliver maximum power.
MPPT control can be applied at the level of an individual PV cell or, with somewhat reduced accuracy and efficiency, to a string of an arbitrary number of series connected PV cells. However, accuracy and efficiency is diminished with increasing numbers of PV cells in a string and, in any case, the power that can be harvested from a string is limited by the reduction of voltage produced by any shaded or partially shaded PV cell in the string. On the other hand, dividing an MPPT controlled string into a plurality of shorter MPPT controlled strings multiplies the number of high accuracy voltage and current sensors and other high-speed circuits of substantial bulk and cost required to minimize effects of shading which may be slight.